The present invention relates to a developing agent used in an image forming apparatus such as an electrostatic copying machine, or a laser beam printer.
The heat roll fixing method is superior to other fixing methods in that an image can be fixed at a high speed, energy efficiency is high, and adverse effects on the environment by, for example, the evaporation of solvents can be suppressed.
On the other hand, it is pointed out as a defect inherent in the heat roll fixing method that so-called “low temperature offset” and “high temperature offset”, i.e., the phenomenon that the toner image is brought into a direct contact with the fixing roll or the belt under a low temperature or a high temperature, tend to easily occur.
The low temperature offset is the phenomenon in which the toner image is not sufficiently melted under a low temperature and, thus, the toner is cracked when the toner image is brought into contact with the fixing roller. On the other hand, the high temperature offset is the phenomenon in which the molten state of the toner is rendered excessive under a high temperature, which causes the toner to adhere to, for example, the fixing roll.
It is desirable for the temperature at which the low temperature offset is generated to be low and for the temperature at which the high temperature offset is generated to be high. Along this line, various ideas have been proposed to date. It has been suggested that the molecular weight distribution of the binder resin should be broadened, and that a low molecular weight propylene series resin having a molecular weight falling within a prescribed range should be added to the binder resin, as a release agent. It has also been proposed that, in order to obtain a toner excellent in the low temperature fixing properties and in the resistance to offset, the toner should contain two kinds of wax components.
On the other hand, it is desirable in view of the energy saving to shorten the waiting time until the temperature of the fixing device is elevated to reach the temperature at which the fixing device is operated and to carry out the fixing operation at a lower temperature. As a method of lowering the fixing temperature, it is known to the art to use a toner containing, as a binder resin, both a crystalline polyester and a amorphous polyester resin, as disclosed in, for example, Jpn. Fat. KOKAI Publication No. 2001-222138.
However, the toners that have been proposed do not satisfy all the requirements and, thus, some problems remain unsolved. For example, some conventional toners, which are excellent in the resistance to the high temperature offset and in the developing performance, are insufficient in the low temperature fixing properties. Also, other conventional toners, which are excellent in the resistance to the low temperature offset and in the low temperature fixing properties, are somewhat unsatisfactory in the resistance to the blocking and in the developing properties. In short, the conventional toners do not simultaneously satisfy the resistance to the offset under low and high temperatures.